A liquid droplet ejection apparatus such as an ink-jet head has been known which conveys ink supplied from an ink tank to eject ink droplets from a nozzle towards a recording sheet. The ink-jet head includes, for example, a flow path unit having nozzles for ejecting ink droplets and a piezoelectric actuator mounted on the flow path unit (see JP-A-10-226095, for example). The flow path unit includes ink flow paths formed for each ink color and nozzles communicating with the ink flow paths. The piezoelectric actuator includes piezoelectric sheets each being sandwiched by a common electrode and a plurality of individual electrodes. Herein, a required individual electrode is applied with voltage to selectively impart pressure to a corresponding ink flow path, so as to generate ink ejecting pressure.
In ink-jet printers, when forming a high-resolution image like a photography-mode image, it is required to eject color ink having a minute liquid droplet diameter. On the other hand, when forming a solid image with black ink over a wide range, it is required to eject black ink having a relatively large liquid droplet diameter so as to reduce occurrence of unevenness in density of the black color on a paper sheet. In order to satisfy these needs, the nozzle diameters may be made to differ from each other by increasing the diameter of the black ink nozzle or decreasing the diameter of the color ink nozzle. It is noted that JP-A-10-226095 describes an ink-jet printer which includes nozzles having different diameters. In the ink-jet printer, even in the case of so-called gradation printing using only the same color ink, droplets having different diameters are ejected so as to execute printing according to any desired printing mode.
In the ink-jet head described in JP-A-10-226095, nozzles have different diameters and in order to eject ink from every nozzle at a same flying speed, driving voltage of the piezoelectric actuator is adjusted for each nozzle diameter. That is to say, the active part length of the piezoelectric element and the width of the individual electrode are varied in accordance with the nozzle diameter. If the piezoelectric actuator has a nonuniform structure like the above-described structure, nonuniform deformation or variation may occur in the process of baking or the like during the manufacturing of the piezoelectric actuator, and therefore, the production cost may increase by the lowering of the yield rate.